Method of catalyzing porous electrodes by replacement plating

ABSTRACT

A porous or sintered fuel cell electrode which is catalyzed by means of a replacement plating process. An acidic plating solution containing a salt of a noble metal catalyst is forced through the pores of a nickel electrode substrate and the noble metal ions from the dissolved salt replace a thin layer of the nickel surface within the pores.

ates

Schulmeister et al.'

11 atent 1 [22] Filed: Feb. 2, 1970 [21] App1.No.: 8,035

[52] US. Cl. 136/120 FC, 117/130 E [51] Int. Cl. H01m 13/04 [58] Fieldof Search. 136/120 FC, 86 D; 117/130 E, 117/119 [56] References CitedUNITED STATES PATENTS 2,409,295 10/1946 Marvin 'et al. 117/119 [1113,787,24 1451 Jan. 22, 1974 3,116,165 12/1963 Hipp 136/120 FC 3,409,47211/1968 Weber et al. 136/120 FC 3,097,974 7/1963 McEvoy et a1 136 120 FC3,167,457 1/1965 Bacon 6131 136/120 FC FOREIGN PATENTS OR APPLICATIONS988,174 4/1965 Great Britain 117/130 E Primary Examiner-L. DewayneRutledge Assistant ExaminerM. J. Andrews [5 7] ABSTRACT A porous orsintered fuel cell electrode which is catalyzed by means of areplacement plating process. An acidic plating solution containing asalt of a noble metal catalyst is forced through the pores of a nickelelectrode substrate and the noble metal ions from the dissolved saltreplace a thin layer of the nickel surface within the pores.

3 Claims, No Drawings Pat.

3,309,231 all disclose such impregnating processes for- METHOD OFCATALYZING POROUS ELECTRODES BY REPLACEMENT PLATING BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to the fieldof fuel cell electrodes such as used in cellscapable of directlygenerating electrical energy through an electrochemical reaction. Moreparticularly, the invention is concerned with a method of manufacturingporous electrodes which contain a catalyst on the active surfaces of theelectrodes to aid the electrochemical reaction.

2. Description of the Prior Art The continuing development of fuel cellshas generated interest in their applications to both scientific andcommercial fields. A fuel cell suitable for both of these fields shouldprovide continued power output with little degradation in performancefor extended periods of time. The desire to increase the capacity of thecell has prompted extensive development in the field of porouselectrodes. The porosity of the electrodes provides a greater surfacearea through which the cell electrolyte can operate and increases theelectrical power output without substantially expanding the physicalsize of the cell.

It is common practice to provide a highly activated surface on the fuelcell electrodes by means of a catalyst which aids the electrochemicalprocess. The catalysts are generally noble metals such as gold, silver,platinum, palladium and other metals of the platinum group. Because ofthe extreme cost of these metals extensive use is economically notfeasible. Economic considerations require that the catalystsbe used insmall quantities in the most efficient way possible. For these reasonscatalysts are generally applied to a substrate material which forms theprincipal structural portion of the electrode. The catalysts are appliedin a very thin film sufficient to catalyze the electrochemical process.

The application of the noble metal catalysts to porous electrodes haselicited a number of new processes for forming thin coatings of anadequate quantity of the catalyst on the electrode substrate. Thecatalysts are frequently impregnated in the pores of the electrode by afiltration process. In these processes the catalysts are pulverized andfiltered or impregnated within the pores by drawing the pulverizedcatalysts through the porous substrate in a differential pressureprocess. These filtering processes require an extremely'fine powder ofthe catalytic material in order to get sufficient coverage of the poresurfaces without clogging the pores. Oxides or salts of the catalystsare ground to a very fine particle size and colloidally suspended in afluid media which is subsequently drawn through the porous substrate.The coated substrates are then exposed to a reducing gas such ashydrogen to reduce the oxide or salt of the noble metal to the puremetallic element which best serves the electrochemical process of thefuel cell. U.S. Nos. 3,097,1l5; 3,097,974; 3,171,757 and porouselectrodes.

Due to the very fine size of the pores in the electrodes, catalyticparticles must be pulverized to a size which is several orders ofmagnitude smaller than the pores to permit adequate coverage of the poresurfaces. This requirement necessitates starting with the catalysts in aform other than the pure metal and a reduction of the metal back to itspure state after it had been dispersed in a thin coating within thepores. Such a process is indirect and requires several chemicalconversions of the catalysts both before and after impregnation toachieve a useful final catalytic coating in the pure form. Furthermore,the impregnation process is frequently followed by a binding process inwhich a binder is superimposed on the deposited particles to provide asecure and durable attachment of the coating with the base material. U.S. Pat. No. 3,171,757 suggests that the binder may be the catalyticmaterial itself and may be deposited by electroplating or chemicaldeposition processes.

It would be desirable therefore to have a more direct process which doesnot require conversion of the catalyst into a powder form and reductionto precipitate the pure metal on the exposed surface of the pores.

Furthermore, it would be desirable to directly attach the catalyst inintimate relationship with the base material without the use of anadditional binder to retain the catalyst in place. An intimaterelationship at the surface of the substrate lowers the internalelectrical resistance of the cell and consequently reduces internalpower losses.

SUMMARY OF THE INVENTION This invention relates to a process of coatinga porous electrode directly with a noble metal catalyst. Moreparticularly, the coating is formed on the surfaces of the poresthroughout the electrode by means of a replacement plating process.

A noble metal catalyst in a salt form, such as a chloride, is dissolvedin an acid solution having a preselected pH factor. The solution is thenforced through the porous metallic electrode substrate. The ions of thecatalyst are interchanged with the metallic ions from the surface of thesubstrate. The replacement process therefore substitutes a catalyst suchas platinum, palladium or gold for the metallic surface of the poreswith the aid of the acid. The metallic ions removed from the substrateare then drawn off with the depleted acid solution.

The substrate material in one embodiment of the invention is a nickelsubstrate which has suitable electrical characteristics and corrosionresistance to provide the electrical output desired of the fuel cell.The nickel also possesses sufficient strength characteristics to providestructural integrity within the cell.

Successful replacement plating with the noble metal catalysts requiresthat the pH factor of the acidic solvent be controlled to provide thecorrect distribution of the catalysts within the pores. In general, thepH factor should lie between 0.5 and 2.0. Too much acidity causes adegradation of the nickel substrate and too little acidity attenuatesthe ion exchange between the catalyst and the substrate.

Proper control of the flow rate of the solution through the poroussinters commensurate with the acidity of the solution is also importantto obtain the correct distribution of the catalyst. Generally, flowrates on the order of 20cc/sec/cm are appropriate for a number ofspecimens in solutions having a-pI-I of 1.0. If the pore size is assmall as 2-3 microns the flow rate of the plating solution may bereduced to 510 cc/sec/cm A corresponding pI-I factor of 2.0 isnecessitated by such small pore sizes to prevent greater plating on theexterior surfaces of the substrate than is possible within the poreswhere removal of the depleted solution is impeded by the finer poresize. Flow rates of the solution and the pH factors must becorrespondingly adjusted according to the porosity and surfaceconditions of the substrate. If an open porous substrate is heavilyoxidized, a pH factor of 0.5 is used in order to adequately reduce theoxide coating prior to plating with the noble metal ions.

Coplating of two or more noble metal catalysts is accomplished byemploying salts of a plurality of catalysts in the plating solution. Thesalts, principally chlorides of platinum, palladium and gold, aredissolved in various concentrations with respect to one another andproportional amounts of the catalysts replace the substrate materialduring the plating process. The multiple plating process permits a morerapid production rate for the electrodes and assures that the differentcatalysts will be intimately joined with low electrical resistance tothe substrate material due to the ionic bonding at the substratesurface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the new and novelmethod for producing a fuel cell electrode with a noble metal catalystintimately joined to the surface of the electrode of the presentinvention, the process is begun by selecting a suitable substratematerial for the electrode. The selection is governed by a number offactors such as the desired operating characteristics and the structuralrequirements of the electrode which in turn may be dictated by theparticular configuration of the fuel cell. Electrodes of planar,cylindrical or other shapes may be catalyzed according to the presentprocess without interferring with the essential objects of theinvention. The base material of the electrode can be selected from thetransition metals which are capable of being plated such as nickel,cobalt and iron. The essential characteristics of the substrate materialaside from its capability of being plated are its high electricalconductivity and corrosion resistance.

Since the plating process relies upon the replacement of the substrateby the ions of the catalysts in an acid solution, it is necessary toselect a salt of the appropriate catalyst which can be dissolved in anacid solution. The frequently used salts for the plating process arechlorides selected from the group consisting of chlorides of platinum,palladium or gold. It is well known that one of the principal propertiesof the noble metals is their extreme stability in the pure form. Thepure metals resist chemical reaction even in the presence of promotersor activators such as acid solutions. It is, in fact, this stabilitywhich permits their use as catalysts in the electrochemical action ofthe fuel cell. Because of the stability and because of the need for afree ion interchange during the plating process, a soluble salt such asa chloride of the noble metal is selected to be mixed with a solventsuch as hydrochloric acid. Chlorides may be dissolved in hydrochloricsolutions having molar concentrations between O.l-l N to obtain theproper plating solution. Other solvents such as nitric acid may beemployed. Generally, a suitable solvent'is the acid which has the sameradical as the salt producing the noble metal ions. The plating solutionis then forced through the porous substrate by applying a pressuredifferential across the substrate in any convenient manner. A typicalexample of this procedure is found in the filtration processes of theprior art where a vacuum is generated on the one side of the substrateto draw the solution through the porous matrix of the base material. Thevacuum process is frequently used due to its simplicity and adaptabilityto electrodes of both planar and cylindrical shapes. If the electrode isplanar, the solution can be forced through the pores by positioning theuntreated electrode in a horizontal position and in sealed relationshipwith the sidewalls of a vertical tank. The solution containing thedissolved salts of the noble metal catalysts is poured on top of theporous electrode and a partial vacuum is drawn on the bottom side of theelectrode. By controlling the pressure of the partial vacuum, the totalpressure differential across the electrode can be adjusted which, inturn, regulates the flow rate of the solution through the porous matrix.

Cylindrical or curved electrodes can be treated in a similar manner bypositioning the electrode coaxially within a vertical cylindrical tank.The ends of the electrode are sealed against the ends of the tank and aconduit fills the inner volume of the electrode with the platingsolution. A partial vacuum is then drawn between the outer surface ofthe electrode and the inner surface of the tank to force the solutionthrough the porous matrix. To insure that the coating is uniformilyapplied at all portions of the cylindrical matrix, the plating solutionis continuously supplied to the inner volume of the electrode tomaintain a continuous flow through the cylindrical electrode at allelevations within the cyindrical tank. If the vertical pressure gradientcaused by the liquid solution within the tank tends to unbalance theflow rates through the matrix at different vertical stations, it may bedesirable to repeat the process with the tank in an inverted position toobtain a uniform coating at all points of the electrode.

The ionic reaction at the interface of the solution and the poresurfaces has been found to depend on a number of factors. If the acidityof the solvent is too strong for the matrix material, a degradation ofthe substrate takes place. For example, it has been found necessary tolimit the pH of the acid to values no less than 0.5 for average poresize sinters in the order of 5-7 microns. Normally a pH of 1.0 issatisfactory. If the acidity is too weak, the substrate resists attackby the acid and, no replacement with the catalytic ions takes place.Acidities weaker than a pH of 2.0 are generally ineffective for theplating of catalytic ions.

Time is another factor related to both the attacking of the substratematerial by the acid and the degree to which the ion exchange takesplace. In order to have a uniform plating on both the exterior surfacesof the substrate as well as the interior surfaces of the matrix, theflow rate of the solution must be held at a relatively high value, onthe order of 20cc/sec/cm for an average pore size sinter. If a smallerflow rate is used, the time at which the internal pore surfaces areexposed to the undepleted solution is too short to permit an adequatecoating of the catalytic elements to form in comparison to the rapid andcontinued reaction which takes place externally of the substrate.

A corresponding reduction in the acidity is called for when the flowrate of the solution is reduced by a smaller pore size, again to inhibitan inordinate amount of plating on the exterior surfaces of thesubstrate matrixes. For example, if the flow rate is reduced from2Occ/sec/cm to l0cc/sec/cm when the pore size is reduced from 5-7microns to 2-3 microns, the pH factor is changed from 0.5-1.0 to 2.0.The precise concentrations would, of course, be established with theflow rates prior to initiation of the plating process but the workablepH range is from 0.5 to 2.0. The change in flow rates referred to doesnot occur during the plating of given substrate but changes from onesubstrate to another depending upon the porosity of the matrix.

Since the acidity controls the degree to which the acid attacks thesubstrate material, it is necessary to vary the acidity for otherconditionsof the substrate surfaces such as the presence of an oxidecoating. Nickel base substrates frequently contain heavy oxide coatingsand require a stronger solution for the initial coating of the catalystthan is necessary for a subsequent coating. The precise thickness of thefinished coat may be determined by successive platings and theresistance of the noble metal catalyst to oxidation requires that theacidity of the plating solution between successive platings be reducedto avoid any degradation of the lightly coated substrate.

The co-plating of the substrate with two or more materials is readilyaccomplished by dissolving salts of the respective catalysts in the sameacid solution. The replacement of a nickel ion, for example, with eithera platinum or a palladium ion can be accomplished with the same acidsolution regardless of the presence of other foreign ions in thesolution. In one example, platinum and palladium were co-plated on anickel substrate from solutions having relative concentrations ofplatinum chloride and palladium chloride which varied over the entirerange of relative compositions. In a similar example, platinum and goldchlorides in various combinations were also co-plated without adverseresuits. As a consequence, the production of multiple coatings can beaccelerated by the simultaneous replacement plating of differentcatalysts.

In accordance with the above teachings, several examples of platingprocesses follow.

EXAMPLE I A nickel substrate of a planar shape was successfully platedwith both platinum and palladium catalysts. The nickel substrate had anarea of approximately 64 square inches, 75-80 percent porosity and poresizes of 5-7 microns. The substrate was placed in a plating solutionformed by hydrochloric acid and salts of both platinum and palladiumchlorides. The ratio of the chlorides was 25 percent by weight platinumchloride and 75 percent by weight palladium chloride. The concentrationof the two salts together provided approximately 5.2 grams of salt perliter of solution. The salt concentrations were based-upon the amount ofcatalyst which was to be deposited on the substrate since the saturationpoint was not exceeded. The pH of the solution was approximately 1.0 andthe solution was forced through the substrate with a flow rate ofapproximately cc/sec./cm. It was found that substantially all of thecatalytic ions were plated onto the substrate when the solution hadchanged from a dark brown color to a light green color indicating thepresence of nickel chloride salt in the solution.

EXAMPLE II The nickel substrate in this case was formed by two layers ofsubstrate secured in side-by-side relationship. The characteristics ofthe one substrate layer were identical to those cited in Example I. Thesecond substrate layer had a porosity of 40 percent and pore sizesranging between 2-3 microns. Due to the reduced porosity of the one ofthe substrate layers, the flow rate of the plating solution through thelaminated substrate structure was l0cc./sec./cm. which required a changein the pH of the solution to 2.0. Otherwise, the solution contained thesame weight ratios of palladium and platinum chlorides and was employedin the same manner as in Example l.

EXAMPLE Ill The nickel substrate in this case was the same as that usedin Example I except that a heavy oxide coating covered the externalsurface of the porous material. The plating solution was also the sameas that employed in Example I except that the pH factor had to bechanged to 0.5. The increased acidity was needed to achieve theappropriate color change from brown to pale green which characterizesthe successful exchange of the nickel and catalytic ions. Solutionshaving a pH of 1.0 and higher numerical values failed to produce thecharacteristic color change when a scaling oxide appeared on thesubstrate surface.

EXAMPLE IV The nickel substrate in this example is essentiallythe sameas that employed in Example I. The plating solution was also the sameexcept that the catalytic ions were formed solely from platinumchloride. The plating solution therefore contained percent by weightplatinum chloride rather than the mixedsalt solution of the previousExamples. It was also noted that no change in the pH factor was requiredwhen the single salt was employed.

Other examples in which the platinum and palladium ions were jointlyplated have also been successfully tested. The ratios of the varioussalts were varied in the different tests by 5 percent incrementsthroughout the complete range between 100 percent of either salt. Instill other examples, gold chloride and platinum chloride were jointlyplated successfully with the same variation in weight ratios. in eachinstance the pH factor of the plating solution did not vary.

It will be understood that while specific examples of the novel platingprocess have been disclosed throughout the specification, the generalteachings of the invention can be employed in various forms withoutdeparting from the spirit of the invention. The replacement plating ofsintered electrodes deposits the catalyst in the pure metallic statewithout requiring subsequent reduction of salts of the catalysts eitherbefore or after coating the matrix material. The process also insuresthat a strong ionic bond with low electrical resistance exists betweenthe substrate and the catalysts even where several catalysts areco-plated. The ability to vary the plating control parameters such asacidity and flow rate of the plating solution facilitates the even distribution of the activating catalytic material throughout the poroussinter and thereby insures an electrochemical reaction within an.operating fuel cell wherever there is an interface of the electrode andelectrolyte.

Having thus described our invention we claim:

ll. A method of catalyzing a porous nickel fuel cell electrode havingpores varying in average size throughout the electrode of from 2-3microns to 5-7 microns, comprising the steps of:

selecting a' plating solution containing at least one salt of a noblemetal catalyst andan acid solvent;

adjusting the acidity of the plating solution to a selected pl-l withinthe range of 0.5 to 2 in accordance with the surface condition and poresize of the electrode;

said flow rate and pH range and pore size being interrelated such thatfor decreasing pore size within the range of from 2-3 microns to 5-7microns the How rate is selected at decreasing rates within said flowrate range of 5cc/sec/cm to 2Occ/sec/cm and the pH is selected atincreasing values within said pH range of from 0.5 to 2, and converselyfor increasing pore size within the range of from 2-3 microns to 5-7microns the flow rate is selected at increasing rates within said flowrate range of 5cc/sec/cm to 2Occ/sec/cm and the pH is selected atdecreasing values within said pH range of from 0.5 to 2; and removingthe depleted plating solution from contact with the electrode at thecompletion of the ion exchange reactionv 2. The method of claim 1wherein the noble metal catalyst salt is selected from the groupconsisting of chlorides of platinum, palladium and gold.

3. The method of claim 2 wherein the plating solution contains salts ofmore than one noble metal catalyst.

2. The method of claim 1 wherein the noble metal catalyst salt isselected from the group consisting of chlorides of platinum, palladiumand gold.
 3. The method of claim 2 wherein the plating solution containssalts of more than one noble metal catalyst.